The chief objective of these studies is to apply observations we have recently made on skin allografts in mice to the problem of rejection or enhancement of tumor transplants. A newly developed method for estimating relative levels of IgG2 and IgG1 of a given antibody in a naturally occurring mixture made it possible to show a relation between the levels of IgG1 and IgG2 anti-H-2 antibodies and the fate of normal skin allografts. The objective of the present work would be to see, in the immune response of mice transplanted with autochthonous tumors, whether a relation can be demonstrated between the levels of anti-tumor antibodies of the IgG1 and IgG2 classes and the enhancement or inhibition of growth of the transplanted tumors. If such relations are found, a further objective will be an attempt to affect the relative production of IgG1 to IgG2 among anti-tumor antibodies by chemical alteration of tumor antigen or by modes of immunization with such antigens. Another objective will be the study of dialyzable preparations derived from tumor antigens by methods recently described for such preparations from normal tissues, where it was found that a dialyzable fraction retained strain specificity. If the dialyzable tumor cell preparations show tumor-antigen specificity, the structure of these will be compared with the analogous preparations from normal mouse tissue of the same strain.